canvas原生实现前端网页移动端签名

let c = document.getElementById("canvas")
let canvas = document.createElement("canvas")
let availHeight = document.documentElement.clientHeight
let off = availHeight - 400
canvas.height = '350'
canvas.width = c.clientWidth
c.appendChild(canvas)
let dr = canvas.getContext('2d')
dr.strokeStyle = 'blue'
canvas.addEventListener('touchstart',(e)=>{
 dr.beginPath()
 dr.moveTo(e.changedTouches["0"].pageX,e.changedTouches["0"].pageY-off)
})
canvas.addEventListener('touchmove',(e)=>{
 dr.lineTo(e.changedTouches[0].pageX, e.changedTouches[0].pageY-off)
 dr.stroke()
})
canvas.addEventListener('touchend',(e)=>{
 dr.closePath()
})

let img = document.querySelector('canvas').toDataURL()